[gn build] Port 491375c

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/SandboxIR/BUILD.gn

@@ -7,6 +7,7 @@ static_library("SandboxIR") {
   ]
   sources = [
     "Argument.cpp",
+    "BasicBlock.cpp",
     "Constant.cpp",
     "Context.cpp",
     "Module.cpp",